The present invention is concerned with a process for preparing PIPA polyols.
PIPA (polyisocyanate polyaddition) polyols have been disclosed before, see e.g. U.S. Pat. Nos. 4,452,923, 4,438,252, 4,554,306, GB 2102822, GB 2072204, WO 94/12553, U.S. Pat. No. 5,292,778 and EP 418039. PIPA polyols are polyaddition reaction products of a polyisocyanate and a low molecular weight compound having a plurality of hydroxyl, primary amine and/or secondary amine groups in the presence of high molecular weight polyols, in particular polyether polyols. The PIPA polyol is a dispersion of particulate material in a polyol and is used e.g. in making slabstock or moulded flexible foams with improved load-bearing properties. The amount of PIPA polyol used in formulations for making such foams conventionally is such that the amount of particulate material calculated on all high molecular weight polyol used in the formulation is 1-15% by weight. The most commonly used PIPA polyol nowadays probably is a PIPA polyol having about 20% by weight of particulate material, which is diluted with further high molecular weight polyol to the above 1-15% by weight loading range.
It would be desirable to be able to provide PIPA polyol with a considerably higher loading. It would allow the foam producer to use PIPA polyol with higher loadings for making the foam. Even if the foam producer would dilute the PIPA polyol with a higher loading, it would have the advantage that the PIPA polyol can be transported in a more concentrated form and can be diluted at the place where it is needed and to the extent needed. Further it provides the polyurethane systems' formulator with less formulation restrictions. The foams made from such PIPA polyols show good fire retardancy properties and are easily recyclable chemically.
Processes for making such PIPA polyols with a higher loading are known, see e.g. the prior art mentioned before. However, these processes lead to products which have a high viscosity and/or are not stable or these processes lead, certainly at a larger scale, to an uncontrollable reaction which gives PIPA polyols which could cause foam collapse when used in making flexible polyurethane foams.
In WO 00/73364 a process is described for preparing a PIPA-polyol having a loading of 30-80% by weight and a relatively low viscosity. The Tg (glass transition temperature) of such PIPA polyols is relatively low and the amount of particles having a particle size of 10 μm and more is rather high, leading to lower storage stability. When used in making flexible foams such PIPA polyols give a cell opening effect which often is too strong and a reinforcing effect which is too low; further the compression set and the fire performance of the foam would need improvement.
The PIPA polyol obtained in the example of WO 00/73364 had a solids content of 50% by weight and a viscosity of 15000 mPa·s at 25° C. The Tg of the particulate material, however, was only 68° C. and up to 15% by volume of the particles had a particle size of more than 10 μm.
Surprisingly, a novel PIPA polyol was found having a higher Tg of the particulate material and a higher volume of the particles having a particle size of 10 μm or less. Such a novel PIPA polyol is made by a combination of measures: 1) the ratio of isocyanate groups and isocyanate-reactive groups in the low molecular weight polyol was increased (in the example of WO 00/73364 this ratio was 61/100 while in the specification a range of 33-99/100 and preferably of 50-80/100 has been proposed; in the process according to the present invention this ratio is 70-100/100 and preferably 75-98/100); 2) emulsification of the polyol having a high molecular weight and the isocyanate-reactive compound having a low molecular weight preferably is conducted at a lower temperature (in WO 00/73364 a temperature of 60-100° C. and preferably of 70-95° C. has been disclosed and 82-85° C. was employed in the example and in the present invention preferably 20-70° C. is employed); 3) temperature control according to the following                during the entire process the temperature may not exceed 150° C.;        during the entire process the temperature may not exceed 120° C. for more than 2 hours and preferably for not more than one hour;        during the addition of the polyisocyanate the temperature is kept at least 10° C. and preferably at least 20° C. and most preferably at least 30° C. above the Tg of the PIPA particle formed at that stage of the process; and finally 4) the addition time of the polyisocyanate preferably is kept as short as possible and is determined by the cooling capacity available so as to keep the temperature within the above given limitations.        
In this respect it is to be realised that the Tg of the PIPA particles increases with the amount of polyisocyanate added almost linearly to about 75-110° C. at the end of the addition.
As such, reinforced polyols having a higher Tg have been disclosed.
For instance U.S. Pat. No. 5,916,994 and U.S. Pat. No. 4,208,314 disclose polymer polyols based on styrene and acrylonitrile (SAN) having a Tg of about 100° C. However, so far no such PIPA polyols have been disclosed. In addition, a narrow particle distribution and a high amount of small particles in the past could most of the time only be obtained following mechanical filtration. The present invention provides for a PIPA polyol with a high Tg and a high amount of small particles without the need of such mechanical filtration.